The present invention relates to the digital encoding of analog displacements and, particularly, to the making of digital measurements of rotational positions of one or more coaxial shafts.
A common type of shaft angle encoder is one where angular position is determined by means of a coded disk that is provided with a number of concentric tracks, each having alternative transparent and opaque means or sectors. In such encoders, a lamp on one side of the disk and a light-responsive cell on the other side confront a point on each coded track. Production of such encoders involves the difficulty of working with transparent material and the expense of producing accurate coding tracks on such material, which is usually glass. In absolute encoders, the multiplicity of lamps and photocells can create cross talk or make it necessary to collimate to avoid dispersal of light around the edges of opaque sectors. In incremental encoders, there is added cost because of the need for extensive means for determining initial shaft position; also, this reliance on initial determination sometimes allows errors to accumulate.
Many of these mechanical, optical and reliability difficulties are eliminated if the disk has a magnetized surface for generating pulses over a limited angular range. Such a disk need not be brittle, needs neither an energy source nor a collimating system, and operates to produce a reliable absolute count.